1. Field of the Invention
One embodiment of the disclosed invention relates to a power storage device.
Note that a power storage device in this specification refers to all elements or devices which have a function of storing electric power.
2. Description of the Related Art
In recent years, with an increase of consciousness about environmental engineering, development of power generation devices using a power generation method which poses fewer burdens on the environment (e.g., solar power generation device) than power generation devices using conventional power generation methods has been actively conducted. Along with development of power generation devices, power storage devices have also been developed.
As a power storage device, a secondary battery such as a lithium-ion secondary battery (also called a lithium ion storage battery, a lithium ion battery, or the like) can be given as example (see Patent Document 1). Lithium-ion secondary batteries have high energy density and are widely popular because they are suited for miniaturization.
In a lithium-ion secondary battery, lithium metal oxide is used for a positive electrode and a carbon material such as graphite is used for a negative electrode. As a positive electrode active material of a lithium-ion secondary battery, for example, a positive electrode active material including composite oxide containing at least alkali metal and transition metal can be given.
In a lithium-ion secondary battery, at the time of charging, lithium in a positive electrode material is ionized into a lithium ion and the lithium ion is moved into a carbon material of a negative electrode material through an electrolyte solution. Generally, when the percentage of a material which ions can enter and leave is increased in an active material with the volume of the active material unchanged, the amount of ions capable of entering and leaving the active material is increased, which can lead to an increase in capacity of a battery.